commons-graph/src/java/org/apache/commons/graph/GraphAlgorithms.java

package org.apache.commons.graph;

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import java.util.*;
import org.apache.commons.graph.algo.*;
import org.apache.commons.graph.exception.*;

/**
 * Collection of graph algorithms such as transposeGraph.
 *
 * @version $Id: GraphAlgorithms.java,v 1.3 2002/01/16 20:38:28 jvanzyl Exp $
 * @author  <A HREF="http://www.inf.fu-berlin.de/~dahm">M. Dahm</A>
 */
public abstract class GraphAlgorithms 
{
  /**
   * Compute the inverse of a graph, i.e. with all edges reverted
   */
  public static final Graph transposeGraph(DirectedGraph g) {
    DirectedGraph graph = new DirectedGraph();

    for(Iterator i=g.getVertices().iterator(); i.hasNext();)
      graph.addVertex((Vertex)i.next());

    for(Iterator i=g.getEdges().iterator(); i.hasNext();) {
      Edge f = (Edge)i.next();

      graph.addEdge(g.getTarget(f), g.getSource(f), f);
    }

    return graph;
  }

    /**
     * Get a Minimum Spanning Forest from a graph.
     */
    public static MinimumSpanningForest 
	getMinimumSpanningForest( Graph g ) {
	return MinimumSpanningForest.getMSF( g );
    }

    public static MinimumSpanningForest 
	getMinimumSpanningForest( Graph g, boolean isMin ) {
	return MinimumSpanningForest.getMSF( g, isMin );
    }

    public static MinimumSpanningForest 
	getMSF( Graph g ) {
	return MinimumSpanningForest.getMSF( g );
    }

    public static MinimumSpanningForest 
	getMSF( Graph g, boolean isMin ) {
	return MinimumSpanningForest.getMSF( g, isMin );
    }

    /**
     * Perform a Topological Sort on a Directed
     * graph.
     *
     * Should throw BackEdgeException if it is not a DAG.
     */
    public static List topoSort( DirectedGraph g ) {
	return Sort.getInstance().topologicalSort( g );
    }
    public static List topologicalSort( DirectedGraph g ) {
	return Sort.getInstance().topologicalSort( g );
    }

    /**
     * Identify all backedges in a directed graph
     * when starting from a particular vertex.
     */
    public static Set getBackEdges( DirectedGraph g, Vertex start )
	throws BackEdgeException
    {
	return BackEdges.getInstance().getBackEdges( g, start );
    }

  /**
   * Test whether the SCC g is a loop.
   *
   * @param super_g super graph containing the subset of vertices
   * @param g subset of super_g (g must be SCC of super_g)
   */
  public static final boolean isLoop(Graph super_g, Graph loop) { 
    return findLoopEntry(super_g, loop) != null;
  }

  /**
   * Test whether the SCC g is a loop.
   *
   * @param super_g super graph containing the subset of vertices
   * @param g subset of super_g (g must be SCC of super_g)
   * @return root node of loop, i.e. the entry point, or null if this is no loop
   */
  public static final Vertex findLoopEntry(Graph super_g, Graph loop) {
    switch(loop.getNoVertices()) {
    case 0:
      return null; // certainly not a loop

    case 1: // singleton must point to itself, i.e. be a self-loop
      Vertex   v     = loop.getVertexArray()[0];
      Vertex[] edges = loop.getVertexArray(v);

      for(int i=0; i < edges.length; i++)
	if(edges[i] == v)
	  return v;

      return null;
    }

    /* Make sure that the root node is the only entry point to the
     * SCC, i.e.  there is no way from a node v \in (super_g \ g) into
     * a node in g except via the root node.
     *
     * So we have to find the root node first ...
     */
    Vertex   root     = null;
    Vertex[] vertices = super_g.getVertexArray();

    for(int i=0; i < vertices.length; i++) {
      Vertex v = vertices[i];

      if(loop.contains(v)) // v is in the SCC, no check needed
	continue;
      
      Vertex[] edges = super_g.getVertexArray(v); // All nodes reachable from v
      for(int j=0; j < edges.length; j++) {
	Vertex u = edges[j];

	if(loop.contains(u))
	  if(root == null)
	    root = u;
	  else
	    return null; // Can't be a loop then, i.e. has more than one root node
      }
    }
  
    return root; // Has loop a root node at all !?
  }
}